Thanks for reporting this. This was a real bug and it has now been fixed for the next Studio release: https://feedback.objo.dev/bug/1149
When SelectionEnabled is True, you can now:
- Drag to select text, including across multiple rendered Markdown blocks.
- Double-click a word to select it, matching standard desktop behaviour.
I have also improved MarkdownViewer support in the visual designer. Its design-time properties (including SelectionEnabled, PreserveScrollPosition, typography, border, spacing and colour settings) are now available in the inspector.